The number of ligament and tendon injuries continues to increase each year. It is estimated, for example, that over 250,000 knee sprains occurred in 1984 alone These injuries also remain difficult to repair and reconstruct. While many would debate about the mechanics of the soft tissue injuries and about the efficacy of surgical treatment of these tissues, most would agree that the surgical results could be improved if the level of force placed on these tendons could be measured and controlled.
Measuring ligament and tendon forces has also been difficult. Many techniques have been tried, some in vivo but most in vitro. These techniques have included use of buckle gages, liquid metal strain gages, Hall sensors and clip gages attached directly to the tissue or adjacent to the tissue's insertion into bone.
Buckle gages have been employed by many investigators studying the characteristics of tendons in vivo; ligaments have been examined both in vitro and more recently in vivo. The buckle gage has a rectangular frame with cross bar between two sides of the frame. The tissue is folded into the buckle frame and the cross bar positioned between the two frame members. As tension increases on the ligament, the buckle frame is deflected producing a output proportional to the tissue load.
While this technique is believed to be most directly related to average axial tissue force and to work reasonably well in long tendons, it has several disadvantages. Buckle gages can induce significant shortening in shorter ligaments and tendons. They can also impinge upon the surrounding soft tissues and bone since they are surface mounted.
Others have attached liquid metal strain gages and Hall effect strain transducers to try and measure forces in tendons and ligaments. These displacement measuring devices have many of the same problems. Each one is surface mounted so that impingement can again be a problem. The liquid metal strain gages contain toxic heavy metals which make them unsuitable for long term in vivo use. These attach to the soft tissue fibers. The Hall effect devices have several problems. First they have no measurable stiffness so it is difficult to correlate their displacement with tissue load. Second, they have a very small linear operating range. Third, with both devices, force must be inferred from tendon or ligament deformation via nonlinear time dependent constitutive properties. Finally, the initial length of these devices when the tissue first develops load must be known before strain and thus stress and force can be determined.
Two other less utilized methods have been described. Anterior cruciate ligament deformation under the tibial insertion site has been reported in two patients. Displacement of the insertion was measured using a strain transducer positioned in a tibial drill hole co-linear with the ligament. The technique unfortunately may compromise the normal ligament attachment to the bone. In the other method, ligament force was measured by recording bone displacement adjacent to the insertion site. Measuring bone strain, however, is demanding because the gages can be difficult to attach near the tissue insertion site. Also a substrate has to be placed on the bone at the insertion possibly compromising the structural integrity of the tissue insertion. Further, gage output during in vitro or in vivo use may come from deformations in surrounding tissues or from weight bearing or muscle forces unrelated to the ligament loads.
While these methods have improved our understanding of tendon and ligament function, a less obtrusive and more direct method is now needed to measure soft tissue force.